A transmitter circuit and a receiver circuit in a high frequency circuit unit of a mobile phone are alternatively connected to a common antenna through a high frequency switch circuit. In the related art, a high electron mobility transistor (HEMT) is used as a switch element in such a high frequency switch circuit. The HEMT is made using a compound semiconductor material. In recent years, the HEMT-type devices have been replaced by a metal oxide semiconductor field-effect transistor (MOSFET) formed on a silicon substrate to reduce cost and size of the high frequency switch circuit. However, for a MOSFET formed on a typical silicon substrate there is a large parasitic capacitance between the substrate and a source/drain electrode, thereby causing the problem of significant loss of power of the high frequency signal.
Increasingly, mobile phones are being equipped to operate in multiple modes and multiple bands, and as a consequence, the number of ports required for a high frequency switch circuit has increased. An increase in the number of ports necessitates increasing the number of data bits in the signal that is required for controlling the connection state (i.e., which ports are connected to the antenna) of the high frequency switch circuit. In the case of a parallel input method in which the control signal is input as a parallel signal, the number of input terminals required is increased in consequence of the increase in the number of ports. In a serial input method, a serial signal is input in synchronization with a clock signal which allows only one data input terminal to be used even if the number of ports is increased. Thus, while the high frequency switch circuit of the related art mainly uses the parallel input method, there is an increasing usage of the serial input method in recent years.
A semiconductor integrated circuit that outputs a data signal in synchronization with the clock signal for use in the serial input method includes an output interface circuit that operates in response to operation of the semiconductor integrated circuit. In general, many other semiconductor integrated circuits are connected to a data bus that is connected to a data terminal used for input and output of data. As such, since many semiconductor integrated circuits are connected to the data bus, the load capacitances of these circuits are required to be taken into account when data is input or output as described above.
In addition, since the data is output in synchronization with the clock signal, a rise time and a fall time of the data signal and delays in the rise time and the fall time are required to be taken into account. The values of the rise time and the fall time are significantly changed by a load capacitance CL and output resistance. For example, the rise time and the fall time are increased if the load capacitance CL is increased. In this case, decreasing the output resistance allows the lengths of the rise time and the fall time to be decreased.
However, if the output resistance is decreased, the rise time and the fall time may be too short in a case where the load capacitance is decreased. If the rise time and the fall time become too short, a problem arises in that high frequency noise components are generated.
Therefore, since the rise time and the fall time are significantly changed depending on the load capacitance in a simple method of decreasing only the output resistance as above described, the ranges of the rise time and the fall time may not be set within a predetermined range for external circuits that have various load capacitances.